1. Field of the Invention
The present invention relates to a wavelength selection filter, a filter unit, a light source device, an optical apparatus and a refractive index sensor, and more specifically to a wavelength selection filter for selectively resonating and reflecting light of a given wavelength contained in incident light, a filter unit having the wavelength selection filter, a light source device having the wavelength selection filter or the filter unit, an optical apparatus including the light source device, and a refractive index sensor having the wavelength selection filter.
2. Description of the Related Art
An optical system or an optical apparatus may require a light beam of a given wavelength alone from a light bundle in which multiple light beams of different wavelengths are mixed together. In this instance, a wavelength selection filter (also called a wavelength filter) is generally used.
Japanese Patent No. 3711446,for example, discloses a wavelength filter including a substrate having fine concave and convex portions at its surface, and a dielectric layer coating the surface having the fine concave and convex portions. In this wavelength filter, the fine concave and convex portions and the dielectric layer form a waveguide layer for light incident on the surface.
Also, Japanese Patent Application Publication No. 2005-275089 discloses a tunable wavelength selection filter that selectively reflects light of a given wavelength, which meets a resonance condition, in incident light as reflected light.
Moreover, Japanese Patent Application Publication No. 2007-232456 discloses a refractive index sensor for detecting the refractive index of an object to be measured.
However, the wavelength filter disclosed in Japanese Patent No. 3711446 has the disadvantage that deviation of the incidence angle of light from a designed value leads to a decrease in the amount of light of a given wavelength reflected by the wavelength filter. Further, the wavelength filter disclosed in Japanese Patent No. 3711446 poses the possibility that the deviation of the incident angle of light from the designed value may lead to reflection of light of multiple wavelengths different from the given wavelength.
Also, the tunable wavelength selection filter disclosed in Japanese Patent Application Publication No. 2005-275089 has the disadvantages of having difficulty in fabrication since fine electrodes have to be formed on a substrate, and also having difficulty in low-cost fabrication since a special material has to be used for a waveguide layer.